Once you have collected them a few times, Leccinum species are for the most part pretty easy to identify to genus. Like other boletes, they are soft-fleshed mycorrhizal partners with trees (and shrubs), featuring tubes and pores on the underside of a cap that sits atop a central stem. Unlike most other boletes, however, their stems are punctuated with scabers which typically become brown or black by the time the mushroom is mature (though in a few species the scabers are light in color, reddish, or nearly invisible to the naked eye). If you are new to bolete identification, compare scabers carefully with glandular dots and reticulation. Aside from the scabrous stems, there is something particularly Leccinumesque about most of the mushrooms in the genus; their stature and colors are somehow distinctive. But while recognizing that a bolete is a Leccinum is usually relatively easy, figuring out what species you have found can be truly frustrating. In fact, if you are a North American collector at this point in time, it is probably not possible to identify most Leccinum species with scientific certainty.

Current research, supported by DNA evidence, points to the probability that the methods we have traditionally used to identify Leccinum species--namely, observation of the mushrooms' physical features with and without a microscope--do not necessarily identify actual genetic species with success. If this reality frustrates you, I'm sorry--but try looking at it this way: this is an exciting time to be collecting Leccinum, and amateur mushroomers and mushroom clubs are in a position to make substantial and important contributions to mycology. Careful documentation of many, many Leccinum collections from across the continent is the key to an eventual mycological understanding of the genus; see Kuo, 2007 ("Mushrooming in the Age of DNA: Now Comes the Fun Part") for further discussion and suggestions.

My treatment of Leccinum (which is far from complete) is more "mycological" and less "field-guide-ish" than other treatments at MushroomExpert.Com, so I apologize to readers who become frustrated with Mycologese, dense technical descriptions, and long lists of references and documentation. I have included a brief, nontechnical "Synopsis" at the top of each species page--but it was, in part, my frustration with the inadequacies in existing North American mycological treatments of the genus that led me to this project, and we will never reach a point where the field-guide descriptions correspond to scientific reality without a revision of the genus on mycological terms.

The fungi in Leccinum are ectomycorrhizal symbionts with members of the Fagales, Pinales, and Ericales (as these orders are currently defined; see Stevens, 2007), primarily with Quercus, Carpinus, Corylus, Betula, Populus, Pinus, Picea, Abies, Tsuga, Pseudotsuga, Arbutus, and Arctostaphylos--and possibly with Salix in the Malpighiales. The vast majority of Leccinum species appear to be very host-specific, limited to association with a given species, genus, family, or, at most, order. However, since data about mycorrhizal association is glaringly insufficient, the precise contours of host specificity are not currently known. At least one species (Leccinum aurantiacum) is known to be a "generalist," able to form mycorrhizae with fairly diverse hosts. Leccinum is best known from the northern hemisphere, but the genus is present in Australia, New Zealand, and Africa and it would be premature to make proclamations about its range before distribution is more thoroughly documented. Collections have come from diverse ecosystems in alpine, boreal, temperate, subtropical, and tropical regions.

On the basis of preliminary evidence (den Bakker and collaborators, 2004b), species of Leccinum appear to have evolved from a common ancestor that was most similar, genetically, to today's oak- and hornbeam-associated species. This anecestral Leccinum may have then split into birch-associated and oak/hornbeam-associated lineages, with the conifer-, Ericales-, and aspen-associated lineages deriving from the birch line. Host switches in the evolutionary history of Leccinum appear to have been followed by periods of rapid speciation--a phenomenon also observed in molecular studies of Suillus, Hebeloma, and Pisolithus (Kretzer and collaborators, 1996; Aanen and collaborators, 2000; Martin and collaborators, 2002).

A generalized morphology of the genus Leccinum has become more difficult to characterize with the addition of DNA data since species of Chamonixia, along with a few species placed by some researchers in Boletus and Tylopilus, must now be characterized as well. However, morphologists have noted affinities between these mushrooms and Leccinum for decades and, excluding Chamonixia, the concept I am using here corresponds almost entirely to that of Singer, published in 1947 and stubbornly reiterated in 1986.

Species of Leccinum are boletoid or gasteroboletoid. With the exception of illustrations and a brief description of Chamonixia caespitosa, I have not treated the gasteroboletoid species here. The boletoid species of Leccinum have a glabrous, fibrillose, or fibrillose-squamulose pileus surface; in some species the margin of the pileus extends substantially to create overhanging flaps of tissue. One rare North American species (Leccinum potteri) has a veil that leaves powdery remnants on the pileus surface. The tube layer is usually depressed around the stipe, and is often swollen in its middle portions. The pores are small and sub-circular. The pore surface is initially whitish or yellowish before the spores mature. The stipe is tapered to the apex, and fairly tough in comparison to the pileus. Its surface is adorned with scabers which may be pale or pigmented initially; pale scabers usually (but not always) darken with maturity. The context is white or pale yellow and may (or may not) discolor when sliced and exposed to air. The pileipellis is disposed as a cutis or a trichoderm, sometimes with inflated or otherwise distinctive elements. Pleurocystidia and cheilocystidia are usually present on the tubes. Scabers are composed of bundled tufts of filamentous hyphae, terminating in caulobasidia and caulocystidia. The spores are smooth and subfusiform.

By this morphological definition, boletoid species of Leccinum are separated from the other bolete genera primarily on the presence of scabers, regardless of whether the scabers darken to brown or black, placing a handful of taxa whose generic position has been hotly debated over the years in Leccinum rather than Boletus or Tylopilus--including Leccinum chromapes, Leccinum longicurvipes, and Leccinum subglabripes.

The bulk of North American Leccinum taxonomy is based solely on morphology, and comes from three publications by Smith, Thiers & Watling (1966, 1967, 1968), from the Smith & Thiers (1971) treatment of the Boletes of Michigan, and from the Thiers (1975) treatment of the Boletes of California. The authors planned a monograph of Leccinum in North America, but never completed it. Thus the publications we have were written as a "provisional treatment, in a sense, to enable our species concepts to be widely tested" and the authors were uncharacteristically lax about "citation of all the material studied, or drawings of the microscopical structures" (1967). Documentation of host preference in these publications is at best imprecise--and at worst, absent. I have examined a smattering of Smith's collection notes and accession book entries (held in MICH), and found no hidden ecological data. I chose Smith's notes for Leccinum longicurvipes as a test case, but there are hundreds of Smith's Leccinum collections in the herbarium. While I doubt that examination of all the collection notes would contribute much in the way of previously hidden significant ecological data, it is of course a remote possibility.

An additional problem for North American Leccinum studies is that many taxa are represented by very few collections--sometimes by only one. While a 30- or 40-year-old exsiccatum from a herbarium can still be studied for micromorphological data, such study is only rarely necessary since Smith and Thiers usually documented the microscopic features of their collections with more rigor and skill than many current researchers (aside from those trained by Smith and Thiers) could possibly duplicate. In the few instances where microscopic data is missing, gaps can be filled in; for example, since Smith & Thiers neglected to describe the caulocystidia of what they were calling "Leccinum griseum," I have examined their collections and added this detail to the record (see Leccinum pseudoscabrum). However, the missing molecular and ecological data may be more difficult or even impossible to recapture. It is not always possible to extract DNA from decades-old herbarium exsiccata with standard (and affordable) methods. As far as recapturing ecological data, the prospects are frequently even more remote. Even when we can make fairly good guesses about collection locations, it is by no means a given that the woods still exist--especially in northern Michigan, where many of the Smith & Thiers Leccinum collections were made and where logging and real estate development have since altered the landscape significantly.

All of this means that a substantial amount of collecting and rigorous documentation must now be done to advance understanding of Leccinum in North America. There is not much to be gained, at this point, in describing "new" species on the basis of whether the context stains pink, then gray when sliced, or whether the hyphae of the pileipellis develop pigment globules in Melzer's reagent. Neither will further alignments of poorly documented GenBank sequences contribute much to our understanding--despite the understandable temptation for molecular biologists to produce such papers in today's publishing climate. The work that is needed is not glorious: collecting specimens; painstaking documentation of ecology; thorough morphological description of collections; the accumulation of hundreds of well documented vouchers in public herbaria for molecular analysis and monographic study. Obviously, this is too much work for one or two Leccinum researchers to accomplish--but it could be accomplished in relatively short order with the help of our continent's mushroom clubs and mycological societies. Several seasons' worth of careful collection and documentation in relatively limited geographic areas where clubs are popular (among them California and the Pacific Northwest, the Rocky Mountains, and the Gulf Coast) would advance Leccinum studies immeasurably; see Kuo 2007 for recommendations.

Ideally we would have, for example, 100 collections of Leccinum rugosiceps with identification supported by molecular and morphological data, and a long list of potential mycorrhizal associates for each collection--which a computer could sift through for common denominators, arriving at a list of a few well supported hosts. I use Leccinum rugosiceps as my example because, while present data suggests fairly convincingly that the species is mycorrhizal with Quercus and that the rigorous ecological documentation I am proposing is not needed, there might easily be a precise host (Quercus rubra? "red oaks" in general?) that has not yet been determined.

In the present study I have consulted Leccinum collections in the extensive online databases of five herbaria (TENN, OSU, MICH, NY, and BPI) for ecological data, but this strategy should be seen as a desperate substitute for working with reliably recorded, carefully documented ecological data--and conclusions drawn from existing herbarium data on ecology should be read as extremely tentative. Even the best and most prominent collectors, many of whose Leccinum collections are in these online databases, did not hold ecology to be as important as it has turned out to be; as a consequence the data regarding potential mycorrhizal associates is minimal and unreliable.

Morphological Characters

Most macromorphological characters in Leccinum should be noted in the field--especially the color of the scabers and the staining reactions of the sliced context. Ideal collections include specimens in all stages of development. In situ photographs from all angles, using diffused flash (see Kuo 2004 for basic techniques with digital cameras) should be accompanied by photographs taken in the laboratory or at home.

Pileus. Color of the pileus, while not negligible, may not be as informative as traditional treatments have supposed. In the European context, den Bakker & Noordeloos (2005) argue that molecular results demonstrate many species are capable of displaying a range of light to dark colors. Texture of the pileus surface also demonstrates a certain amount of variability, though some species are consistently fibrillose-squamulose, glabrous, etc. The margin of the pileus usually projects at least a millimeter or so beyond the tubes when the mushroom is young, but by maturity species in subsection Leccinum display substantial overhanging marginal flaps of tissue. One North American taxon, Leccinum potteri, features veil remnants on the young pileus surface.

Hymenium. The color of the tubes and pore surface in both young and mature specimens should be recorded, together with bruising and staining reactions. Standard measurements (depth of the tubes, number of pores per mm) should be taken from mature specimens.

Stipe. Since proportions of the stipe may have some relevance in the North American context (for example, in separating Leccinum crocipodium), diameter measurements should be taken from young and mature specimens at the apex, the widest point, and the base (the point of insertion into the substrate). The "ground color" beneath the ornamentation should be recorded, together with textural surface details (many species develop soft ridges or even quasi-reticulation, aside from the scabers). Some species demonstrate bluish or olive stains on the stipe surface, usually in the basal region. The size ("fine" or "coarse") and density of the scabers should be assessed for the upper stem, the midportion, and the basal area. Scaber color across the developmental stages has proved to be very important in the European context, and the present treatment holds this character (together with its microscopic corollary, the staining reaction of caulocystidia in KOH) as a primary hypothesis for potential morphological separation of North American material. Since the contemporary concept of Leccinum has freed us from the debate over whether darkening of the scabers to dark brown or black is indicative of the genus, we may now be able to describe scaber color more objectively; with no disrespect intended, I suggest that some of the Smith & Thiers descriptions of scaber color (especially those supported by one or a few basidiocarps) may have been unconsciously loaded with the need to justify their position in their argument with Singer, resulting in darker mature scaber colors (or assumed darker colors). The color of the scabers in both buttons and mature specimens should be recorded--but "[c]are should be taken in using this character when the fruit-bodies are found in high vegetation" (den Bakker & Noordeloos 2005) since grass, tall sphagnum, or substrate material can keep light from the scabers and affect whether they darken or not.

Context. The color of the context in Leccinum ranges from white to pale yellow. Staining of the sliced context, while not irrelevant, has probably been given more taxonomic priority in the North American context than variation (and consistency in methodology) should allow. In particular, the presence of a pinkish staining stage before the stained surface resolves to grayish or black has likely been overemphasized. In the European context, staining and the eagerness of the staining reaction have been found to be occasionally informative, but generally so only in limited taxonomic areas (for example the consistently weak staining of Leccinum vulpinum). Some species usually display bluing flesh in the stipe base or in the pileus, but the bluing may not be consistent enough to justify taxonomic priority. Staining reactions should be documented at the time of collection when the mushrooms are fresh.

Spore Print. While spore print color has been used as a predictive character frequently in traditional treatments of Leccinum, it has also been the source of considerable confusion resulting from the difference between a "wet" and a "dry" print--and the character has occasionally been "fudged," as in Smith, Thiers & Watling (1966), where spore print color plays a major role in the key but taxa with undocumented print colors are found subsequent to choices emphasizing the character. In the European context den Bakker & Noordeloos (2005) do not even bother to record or discuss spore print color. While I doubt that the character has any predictive value in the North American context, either, it may be worth continuing to record the print color and preserving prints along with exsiccata--at least until substantial research documents the potential futility of the effort.

Odor and Taste. To my knowledge odor and taste, which are almost universally "not distinctive" in the genus, have not been found to have substantial variation or predictive value within Leccinum.

Exsiccata. Examination of the macromorphology of exsiccata falls into two categories: attempting to recapture details the specimen manifested in the fresh state, and assessing macromorphological features that result from the drying process. As for the first category, it is obviously preferable to examine a fresh specimen or photographs of the specimen in the fresh state, and macromorphological observations based solely on examination of exsiccata should be seen as tentative. In the second category, features resulting from the drying process that may have some predictive value in limited taxonomic areas include the color of the pileus, hymenium, scabers, and context as dried--but variation in drying methods and the age of herbarium specimens may influence these features.

Micromorphological characters that appear to have predictive value in some taxonomic areas of Leccinum (when placed in ecological and molecular context) are derived from observation of the anatomy of the pileipellis, the hymenium, the stipitipellis, and the basidiospores. For the latter, a spore print is the ideal source of material and a water mount will suffice. A radial section of the pileus surface containing about 1 mm of the pileus context for visual orientation should be mounted in unstained, 2-5% KOH or in water. The hymenium should be sectioned perpendicular to the tubes and mounted in unstained KOH. The stipitipellis should be sectioned by slicing a paper-thin section in the mid-portion of the stipe, parallel to the stipe surface, including at least one scaber (in specimens where scabers are clearly defined), and mounted in unstained KOH. If a spore print is unavailable, the hymenium and stipitipellis sections will provide spores for observation if the material was mature. When examining exsiccata, I have found these sections to be more easily accomplished without preliminary rehydration in alcohol and water--and I have compared results from rehydrated sections and straight-to-KOH sections without finding any differences.

Pileipellis. In the European context, den Bakker & Noordeloos (2005) found "two basic types" of pileipellis in Leccinum: a "trichoderm with erect chains of elements" and a "complex type of intricate trichoderm, often with a cutis-like suprapellis." For the sake of convenience the authors call these types "trichoderm" and "cutis"; I have also used this terminology. Additionally I have used the admittedly imprecise term "epithelium" to refer to a trichoderm-like pileipellis in which terminal elements are substantially swollen. Precise documentation of the anatomy of the terminal elements of a trichoderm-like pileipellis may have some value in the North American context; Leccinum rugosiceps, for example, appears to manifest swollen penultimate and antepenultimate elements with consistency. However, the presence or absence of "cylindrocysts" (see Lannoy & Estades 1995), together with the formation of pigment globules in a Melzer's reagent mount (see Smith & Thiers 1966), represent characters that appear not to occur with reliable consistency, and can now safely go unrecorded.

Hymenial Cystidia. Den Bakker & Noordeloos (2005) found "no diagnostic value" in the morphology of hymenial cystidia among European Leccinum species; they found shapes and sizes to vary without consistent correlation to genetic species. However, data on hymenial cystidia is inconsistently recorded in the den Bakker & Noordeloos treatment, and the color of the cystidia in a KOH mount (particularly the cheilocystidia on the pore surface, which tend to display more pigmentation) may turn out to have limited predictive value (perhaps paralleling the KOH reaction of the caulocystidia) and should not yet, in my opinion, be discarded as a potentially valuable character.

Basidiospores. While careful documentation of basidiospore morphology should be continued in North American Leccinum studies, we should probably not expect the data to reveal much for potential species diagnosis. Spores in Leccinum are often quite variable in size, and exceptional "monster" spores are not infrequently encountered. Additionally, the current data on spore morphology in North American Leccinum cannot be relied upon, due to inconsistencies in methods: immature spores have undoubtedly been included; little or no effort has been made to account for potential differences in dimensions of spores produced in the tubes or on the stipe; mounting media have been inconsistent and poorly documented. In the European context den Bakker & Noordeloos (2005) found spore shape (not length, in and of itself) to have some potential value as a character; researchers who want to generate basidiospore morphology data that can successfully be compared to the data of den Bakker & Noordeloos should follow the same methods:

"Spores were mounted in demineralised water and observed under oil-immersion. Only spores from the (pileal) hymenium were used and an attempt was made to measure only mature spores. Spores were considered mature when a clearly developed, slightly (brownish) coloured spore wall was present and guttules could be observed within the spore. Sometimes extremely elongate spores (Q > 4) were present in the hymenium of older fruit-bodies. These are considered anomalies, probably associated with unnatural aging of the fruit-bodies. These were not included in the measurements. Circa 30 spores per collection were measured."

Caulocystidia. Caulocystidia in Leccinum vary substantially, and in most species a wide variety of cystidia shapes can be demonstrated. However, in some species one or a few types of caulocystidia represent the overwhelming majority, and are easily demonstrated without a painstaking search. Additionally, the color of caulocystidia as mounted in KOH appears to have some consistency and, consequently, diagnostic value in limited taxonomic areas. Relatively little attention has been paid to caulocystidia by North American (or other) researchers--but since caulocystidia represent, in part, a microscopic corollary to the scabers, which den Bakker & Noordeloos (2005) have found to be substantially more worthy of attention than was previously suspected, it is worth maintaining the hypothesis that careful documentation of caulocystidia may help support identification--a hypothesis used by den Bakker & Noordeloos with success in the European context.

Macrochemical Reactions

Whether or not macrochemical reactions have any predictive value in Leccinum cannot currently be assessed, since data is inconsistently and sparsely recorded throughout even the morphology-based literature. Apparently distinctive reactions--like the bright red reaction of the pileus surface to KOH in Leccinum longicurvipes--which prompted M. E. Bigelow to write "fantastic!!" when annotating a 1969 collection (HEB 15796 in NY)--must now be documented again and again, and confirmed by ecology- and molecular-based research. Reactions to potassium hydroxide (a 5-20% aqueous solution), ammonia, and iron salts should be recorded in the field, and the chemicals should be applied to the pileus surface, the context (in the stipe, in the pileus), the pore surface, and the stipe surface (including the scabers). Ideally, reactions should be recorded for both young and mature specimens. I believe the data for macrochemical reactions is worth recording, at least for the time being--though the subtle gradations in the reaction of the context to iron salts were not found to be informative (and possibly subject to "age and humidity of the fruit-body") for European taxa in subsection Scabra by den Bakker & Noordeloos (2005).

Under various hardwoods and conifers (apparently a generalist) east of the Rocky Mountains; scabers pink to red at maturity; stem base becoming chrome yellow within and without; cap pink becoming pinkish tan.

Under oaks, birch, or aspen; scabers inconspicuous (often only apparent with a hand lens or after drying); flesh yellow, not changing when sliced or changing slightly to reddish or pale blue; pileipellis a trichoderm with inflated terminal elements (an "epithelium").

Under various oaks east of the Great Plains, southward through Texas and Mexico; fresh cap brown, yellow, or somewhere between; young pore surface dull to bright yellow; stem whitish, brownish, or yellowish underneath pinkish to reddish brown or brown scabers; flesh in base of stem never bluing when sliced.

10

10.

Pore surface bright yellow, at least when young; stem frequently swollen in the middle, with a pinched base; stem surface usually yellowish, often by maturity with scabers arranged in a network of low ridges (reminiscent of a broad reticulum).

Under birch, aspen, or other hardwoods--or under members of the Ericales (madrone, manzanita, huckleberry bushes, kinnikinnick, etc.) . . . or growing in coastal sand dunes, apparently with grasses or sedges.

Growing in coastal sand dunes, apparently associated with beach grasses or sedges; cap orangish; scabers "hazel to vinaceous buff"; flesh pinkish, then grayish or purple-gray when sliced; caulocystidia clavate, grayish in KOH; known from New Brunswick.

Leccinum arenicolaRedhead & Watling, 1979(CJB 57: 117-119)

14.

Not completely as above . . .

As of April, 2007, I have not finished investigating most of the remaining North American taxa, which constitute the bulk of Leccinum. I have placed a few taxa (linked below) in the "uncertain" category, and I have treated the white, birch-loving Leccinum holopus--but most of the work remains to be done. I apologize to readers who are disappointed and are eager to identify collections made under manzanita, madrone, birch, and aspen--but in my defense I will say that the work published here represents most of my available time in the winter of 2006-2007, and I hope to return to Leccinum after another season of collecting mushrooms. Below is a taxonomic digest of North American Leccinum (taxa that I have treated are linked, and links are provided when texts are online), and the references list.

Den Bakker, H. C., B. Gravendeel & T. W. Kuyper (2004a). An ITS phylogeny of Leccinum and an analysis of the evolution of minisatellite-like sequences within ITS1. Mycologia 96: 102-118. This paper is available online here, at Mycologia.

Kuo, M. (2007). Mushrooming in the age of DNA: Now comes the fun part. Retrieved May 2, 2007 from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/kuo_08.html; forthcoming in McIlvainea (in press).